The Science of Existence (13) Dust


The primordial cosmos consisted of hydrogen and helium, with slight traces of lithium. The heavier elements – needed for planets and everything else made of matter as we think of it – are manufactured via supernova explosions. Such star dust was sucked into new stars as they formed. Hence, ever-heavier elements were made.

The first supernova stars had already lived their lives and were gone 13.62 billion years ago. By 13.22 bya, the relentless progression of dust trade-up was well underway.

(There are considerable discrepancies in chronologies among the various accounts of cosmic evolution, all of which are highly speculative. On the previous page, the first stars formed 13.72–13.27 billion years ago; now, a mere page later, supernova stars had already blown their lights out 13.62 bya.)

Even now, most of the ordinary matter in the cosmos is hydrogen and helium. The most abundant molecule in the universe – H2 – primarily forms on the surfaces of dust grains. Heavier elements make up only 1% of galactic mass. Half of heavy matter is bound in dust grains which are blown into existence by the aftermath of a supernova; thus, from dust to dust.

Dust largely defines the interstellar medium. In absorbing ultraviolet radiation from stars, dust emits electrons that are the main heat source of interstellar gas. This bit of warmth from dust helps molecules survive the harshness of deep space. Hence, cold, diffuse clouds of molecular hydrogen course the vastness of space. These clouds may be as frigid as 7 K and as diffuse as 300 light-years.

Over billions of years, the persistent gentle nudge of gravity corrals the molecules in interstellar space closer to each other. They warm as they snuggle. The excitation hastens further condensation. Stars form in these regions. Dust is the subtle conductor of star formation.

Absorbing radiation imparts momentum to dust grains, driving them away from newly formed stars, or even an entire galaxy. Such winds transfer vast amounts of matter between galaxies. Large galaxies, such as the Milky Way, may have amassed half their matter from neighboring star clusters up to a million light-years away. Thus, dust plays an essential role in the evolution of galaxies.

Dust dies by shock: destroyed by shock waves that emanate from the remnants of a supernova. The shock waves are partly comprised of high-speed dust grains, traveling in excess of 1,000 km second. These fast-moving grains are also subject to shocks as they come to rest in the interstellar medium. When it comes to cosmic dust, what comes around goes around.